move the asteroid width bound into FactoryState

It was the last piece of mutable world data BuildingSystem still owned, and the
placement queries need it: isPlacementValid reaches it through
bodyCellsWithinWorldBounds, so those queries cannot become free functions over
FactoryState while the bound lives on the system.

Left out of the checksum deliberately. It is derived from config and Simulation's
expansion count, which is folded already, so adding it would change every
checksum without adding information.

Still seeded from config by BuildingSystem's constructor, which keeps the
initialization at exactly the point it happened before; reset() clears the state
before initializeSubsystems() rebuilds the system, so the ordering holds.

Verified with a golden-checksum capture before and after — all four sample ticks
identical.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/lib/sim/BuildingSystem.cpp

@@ -40,8 +40,8 @@ BuildingSystem::BuildingSystem(const GameConfig& config,
     , m_spawnShip(std::move(spawnShip))
     , m_isItemUnlocked(std::move(isItemUnlocked))
     , m_rng(rng)
-    , m_asteroidWidth_tiles(config.world.regions.asteroidWidth_tiles)
 {
+    m_state.asteroidWidth_tiles = config.world.regions.asteroidWidth_tiles;
 }
 
 // ---------------------------------------------------------------------------
@@ -340,7 +340,7 @@ bool BuildingSystem::bodyCellsWithinWorldBounds(const std::vector<QPoint>& bodyC
                                                 QPoint anchor) const
 {
     const int heightTiles = m_config.world.heightTiles;
-    const int leftEdgeX   = -m_asteroidWidth_tiles;
+    const int leftEdgeX   = -m_state.asteroidWidth_tiles;
     for (const QPoint& cell : bodyCells)
     {
         const QPoint worldCell = anchor + cell;